Due to the requirements for high capacity, high recording density, and fast access for a magnetic disc drive (HDD) in recent years, a magnetic disc is inevitably rotated at a high speed and a head gimbal assembly (HGA) is driven at a high speed. Therefore, no small quantities of turbulences of air (wind turbulences) occur, and vibrations are generated to the disc and head gimbal assembly. The vibration due to wind turbulence greatly impedes the positioning of a head on the information on a disc recording high density. This is because the wind turbulence occurs randomly and it is difficult to make accurate estimate of the size and period, so that rapid and accurate positioning control becomes complicated and difficult. The wind turbulence vibration is also a primary factor of noise and becomes a main cause of impairing the silence of a drive.
As a problem occurring by the action of air in a disc drive accompanied by high speed rotation, there is also the increase in consumption of electric power. When a disc is rotated at a high speed, air in the vicinity of the disc is also drawn together and rotated. On the other hand, air apart from the disc stands still, so that shear force is generated between them, which applies load when disc rotation is stopped. This is designated as windage loss, and the higher the rotation speed, the greater the windage loss. To perform high speed rotation against the windage loss requires great output of the motor and high electric power is necessary.
Since the wind turbulence and the windage loss are proportional to the density of gas in a disc drive, there has been an idea of trying to reduce the wind turbulence and the windage loss by filling and sealing a magnetic disc drive with the gas lower in density than air, in place of air.
As low density gases, hydrogen and helium gases are proposed, but helium that is high in effect and stability is optimal from the practical use. With a magnetic disc drive sealing up helium gas hermetically therein, the above problems can be solved, and rapid and accurate positioning control, saving of electric power, and good silence can be realized.
However, the molecules of helium are extremely minute and the diffusion coefficient is great, so that the low sealing property in casings used in an ordinary magnetic disc drive will cause the helium to easily leak out during general use.
Accordingly, for the purpose of making low density gas such as easily leaking helium hermetically sealable, a sealing structure, e.g., the one as disclosed in U.S. Patent Publication No. 2005/0068666 (“Patent Document 1”) is proposed. FIG. 7 is a cross sectional view showing the structure of the casing of the magnetic disc drive disclosed in Patent Document 1. Casing 100 has base 120 and cover 110 joined with the upper parts of the sidewalls of base 120, and head disc assembly (HDA) 101 is encased in casing interior 102. For sealing up helium in casing interior 102, attaching of cover 110 is performed in the helium gas atmosphere and a sealing type magnetic disc drive having been filled with helium in casing interior 102 is obtained at the same time with attaching.
As the place where there is a high possibility of leaking of the helium in the casing, joint 105 of the upper part of the sidewall of base 120 and cover 110 is exemplified. For completely sealing joint 105, cover 110 is welded by laser on the upper parts of the sidewalls of base 120.
From the viewpoint of durability and reliability, as the base and cover, a base molded by aluminum die casting, and an aluminum cover molded by pressing or cutting are selected.
However, when a base is molded by die casting, a mold releasing agent is coated on the mold for easy release, and gases in the atmosphere are present in the mold. In casting of a base, impurities such as the mold releasing agent and gases in the atmosphere are drawn and contained in a molded base. It is necessary to join a cover and such a base in a state of mounting the HDA. As a joining method, laser welding, soldering or brazing is proposed. By soldering or brazing, the temperature at the joint is lower than the case by laser welding, and the range of the base to be fused is far shallower as compared with laser welding, so that the interior of the base is hardly adversely influenced as compared with the case of the later described laser welding. However, the joint temperature is liable to diffuse by soldering or brazing and a wide range of the periphery becomes high in temperature, so that there is a possibility that the HDA may break from the problem of the heat resistance of HDA.
Further, in laser welding, the welding part alone becomes high temperature, and the HDA is not adversely influenced as in the case of soldering or brazing. However, in laser welding, the joint temperature is higher than in the case of soldering or brazing, and the range of the base to be fused is far deeper as compared with soldering or brazing, so that the interior of the base is adversely influenced. Specifically, as described above, impurities are contained in the base molded by die casting, and the boiling point of these impurities is lower than that of aluminum. Therefore, when the fused part of a base becomes high in temperature, the impurities suddenly expand prior to aluminum and blow off the aluminum in the periphery, as a result a defect (a blowhole) occurs. Sealed up helium is liable to easily leak through such a blowhole.
In summary, when a cover is joined with the base of a magnetic disc drive by welding, bubbles in the die casting suddenly expand due to high temperature at the time of welding and blow off fused metal, and a blowhole occurs. Low density gas sealed up in a casing is liable to easily leak through a leak path made by joining of blowholes.